aspect effects
DESCRIPTION
ASPECT EFFECTS. Photosynthetically-active radiation (spectral portion,0.3-0.4 CI). 0400-0500h. 0500-0600h. 0600-0700h. 0700-0800h. 0800-0900h. 0900-1000h. 1000-1100h. 1100-1200h. TERRESTRIAL. RADIATION. Longwave Radiative Exchange. The atmosphere absorbs long-wave radiation (L) - PowerPoint PPT PresentationTRANSCRIPT
Photosynthetically-active radiation(spectral portion,0.3-0.4 CI)
0400-0500h 0500-0600h 0600-0700h 0700-0800h
0800-0900h 0900-1000h 1000-1100h 1100-1200h
Figure 5.4 Top-of-atmosphere solar radiation on plane surfaces of 45 degree slope at 2.3°N, 77.0ºW, on Julian Day 352.
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Aspect
Solar radiation(W/m2)
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Radiation receipt varies considerably inany mountainous environment
TOA K onA 45° slopeOn Julian Day352, 2.3N
Longwave Radiative Exchange
The atmosphere absorbs long-wave radiation (L) from the Earth, clouds and gases at all altitudes
Absorption greatest in lower portion of the atmosphere,where H20 and CO2 concentrations are highest
The atmosphere absorbs effectively from 3-100 m,except in the atmospheric window (8-11 m)
Most longwave loss to space occurs through this window, but clouds can partially close it
L = 0 (T0)4 + (1 - 0) L
Amount of L reflected(slight adjustment)
L is greater in magnitude and more variable than L
L* = L - L (usually negative)
NET ALL_WAVE RADIATION
DAYTIME: Q* = K - K + L - LQ* = K* + L*
NIGHT: Q* = L*
Radiation Measurements
L
L K
K (not visible)
UV-A
PAR
More radiation sensors…
Source: University of Colorado
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S o la r T im e
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K in tropical forests of Colombia/Ecuador
Radiation Balance Components
Negative in Oke
Clouds
Reduce K because of absorption and reflectionfrom cloud tops (may eliminate S)
Increase D by scattering incoming solar radiation
Strongest K under partly cloudy skies with sunin clear patch
Absorb much of L and re-emit it as L(low cloud emits more)
Reduce diurnal temperature variation
Source: NOAA
GlobalEnergyBalance(SIMPLIFIED)
Q* - positive in daytime- almost always negative at night
Any Q* imbalance is accounted for byconvective exchange or conduction
Q* = QH + QE + QG + S
where QH = sensible heat fluxQE = latent heat fluxQG = conduction to or from ground
(See Figure 1.10)
Conduction, Convection and Advection
Conduction is the process through which heat is diffused to cooler materials as radiation is absorbed. Land surfaces heat quickly,while water bodies can mix and have higher heat capacity. Solids(land) are better conductors than gases (atmosphere).
Convection is physical mixing with a strong vertical motion ingaseous or liquid media. As heat is absorbed by the ground, the airimmediately above is heated. Warm air is less dense and, thus,rises, while cooler air falls.
Advection is the term used to describe lateral heat transfers. Windscarry heat from absorbed radiation from one area to another.
Recall the First Law of Thermodynamics
ENERGY IN = ENERGY OUT
Qin > Qout (flux convergence)Net storage gain leads to warming
Qout > Qin (flux divergence)Net storage energy loss leads to cooling
Qin = Qout
No net change in energy storage
Water: H2O•High heat capacity•Exists in all states at Earth’s temperatures
•Heat required/released during phase changes:
Latent heat of fusion (Lf = 0.334 MJ kg-1)Latent heat of vaporization (Lv = 2.45 MJ kg-1)Latent heat of sublimation (Ls = Lf + Lv)
Water Balancep = E + r + s
Where p is precipitationE is evapotranspirationr is net runoffs is soil moisture storage content
QE = Lv EQM = Lf M
Where E and M are in kg m-2 s-1See Fig. 1.13
Sensible and Latent Heat Fluxes
Eddy correlation (later)
•Sonic anemometer measurements of vertical velocity and temperature
•Krypton hygrometer measurements of water vapour density
Advection and Winds
Air flow at local scale can affect energy balanceas can air flow at scales larger than boundary layer
At the micro-scale, horizontal temperature variation causes horizontal pressure differences
Why ? Warm air is lighter than cold air
This leads to winds (kinetic energy)
Energy transferred to smaller and smaller scales before being dissipated as heat
DAYTIME:
• Both sides of equation are positive: surface radiative surplus
• Surplus partitioned into ground and atmosphere
Convection is the most important means of daytime heat transport from surface
QE is greater when soil moisture is highQH is greater when water is more restricted
NIGHT:
• Both sides of equation are negative:surface radiative deficit
• Deficit partitioned into heat gain from groundand atmosphere
Q* loss is partially replenished by QG
QE and QH of less importance as convectiveexchange is dampened by the night-timetemperature stratification